Tapered block copolymers

ABSTRACT

Disclosed are bituminous blends having improved toughness and elasticity. The blends contain a bituminous binder and a tapered block copolymer formed from a monovinyl aromatic compound and a conjugated diene. Suitable tapered block copolymers include a tapered A-B-A type block copolymer of the A-t-B-t-A configuration, particularly those having an isolated monovinyl aromatic compound content of about 20 to about 90 weight percent, and tapered A-B-A type block copolymers of the A-B-t-A configuration having an isolated monovinyl aromatic compound content of about 20 to about 90 weight percent of the total monovinyl aromatic compound in the tapered block copolymer. Further disclosed is a method of making the blends. Further disclosed is a tapered A-B-A type block copolymer having an isolated monovinyl aromatic compound content of about 29 to about 75 weight percent.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. Ser. No. 647,954, filed Jan. 30, 1991, now U.S. Pat. No. 5,100,939, which is a continuation-in-part application of co-pending applications U.S. Ser. No. 427,431 filed Oct. 27, 1989 now abandoned, and U.S. Ser. No. 502,970 filed Apr. 2, 1990, now abandoned, which are also incorporated herein by reference.

BACKGROUND OF THE INVENTION

The use of petroleum residuum such as asphalt as a paving material, crack sealant, chip sealant, roofing material and the like is well known. It is also well known that the physical properties of such asphalts can be modified by blending various polymeric materials, such as polyethylene, and elastomers, such as styrene-butadiene-styrene block copolymers with the asphalt.

The present invention is directed to asphalt blends containing specific block copolymers having improved toughness and elasticity compared to known blends employing tests well known in the art for comparative purposes.

SUMMARY OF THE INVENTION

A bituminous blend composition according to the present invention comprises a bituminous binder and from about 1 to about 18 weight percent of a tapered A-B-A block copolymer of the A-t-B-t-A configuration based upon the weight of the tapered block copolymer and the binder. The A block is formed from a monovinyl aromatic compound and the B block is formed from a tapered copolymer of a conjugated diene and the monovinyl aromatic compound. Optionally, a portion of the tapered copolymer may comprise a polymer of the conjugated diene.

A bituminous blend composition according to the present invention comprises a bituminous binder and from about 1 to about 18 weight percent of a tapered A-B-A block copolymer of the A-B-t-A configuration based upon the weight of the tapered block copolymer and the binder wherein the tapered block copolymer has an isolated monovinyl aromatic content of between 20 and 90 weight percent based upon the total monovinyl aromatic compound in the tapered block copolymer. The composition of the A block and the B block is as described above.

A method for forming a bituminous blend composition according to the present invention comprises blending a bituminous binder at an elevated temperature with from about 1 to about 18 weight percent of the tapered block copolymers described above based upon the weight of the tapered block copolymer and the binder.

A tapered A-B-A block copolymer according to the present invention comprises A blocks formed from a monovinyl aromatic compound, a B block formed from a tapered copolymer of a conjugated diene and the monovinyl aromatic compound, a portion of the tapered block copolymer optionally comprising a polymer of the conjugated diene.

DETAILED DESCRIPTION OF THE INVENTION

A "tapered" A-B-A block copolymer of the A-t-B-t-A configuration is a block copolymer wherein the B block contains a gradual and continuous change of composition from A rich to B rich or less A rich and then back to A rich, as compared with an A-B-A configuration having an essentially pure A block connected directly to essentially pure B block and then to an essentially pure A block again. The tapered block copolymer of the A-t-B-t-A configuration may be distinguished from the entirely pure block copolymer configurations e.g., A-B or A-B-A or only one sided tapered block copolymers e.g., A-B-t-A or A-t-B.

A "tapered" A-B-A block copolymer of the A-B-t-A configuration is a block copolymer wherein an A block is connected directly to a B block or the B-rich portion of the B block, which have a gradual and continuous change of composition from B rich to another A block. The molecular weights of the two A blocks may or may not be identical. The tapered block copolymer of the A-B-t-A configuration may be distinguished from the entirely pure block copolymer configurations e.g., A-B or A-B-A, other one-sided tapered block copolymers such as A-t-B, or two-sided tapered block copolymers such as A-t-B-t-A.

Particularly useful tapered A-B-A block copolymers are those having an isolated monovinyl aromatic monomer content of about 20 to about 90, more preferably of about 25 to about 80, and most preferably about 29 to about 75 weight percent of the total monovinyl aromatic compound in the tapered block copolymer. Isolated monovinyl aromatic compound content is a measure of the degree of taperedness: the higher the content, the higher the degree of taperedness. Isolated monovinyl aromatic monomer content is measured by proton nmr in accordance with the method of Moehel, Rubber Chemistry and Technology, vol. 40, p. 1200, 1967. Tapered block copolymers having an isolated monovinyl aromatic compound content of 20 percent or greater may be referred to as "enhanced" tapered block copolymers. Desirable enhanced tapered block copolymers include those of the A-t-B-t-A and A-B-t-A.

The tapered A-B-A block copolymers comprise from about 5 to about 50 weight percent of the monovinyl aromatic portion (preferably about 15 to about 40%) and from about 95 to about 50 weight percent (preferably about 85 to about 60%) of the diene portion.

The A block comprises a polymer formed from a monovinyl aromatic monomer of the formula: ##STR1## wherein n is an integer of 0 to 3, R₁ is individually selected from the group of alkyl radicals containing up to about 5 carbon atoms and R₂ is H-- or CH₃ -- and mixtures thereof. Such compounds include styrene, α-methylstyrene, vinyl toluene (all isomers alone or in admixture), ethylstyrene, dimethylstyrene, trimethylstyrene, tertiary butylstyrene, tertiary amylstyrene, and isopropylstyrene. The A blocks may be comprised of the same or different monovinyl aromatic compounds.

The B block comprises a tapered copolymer of a conjugated diene and the monovinyl aromatic compound wherein a portion of the tapered copolymer optionally comprises a polymer of the conjugated diene. The taperedness of the B block allows a symmetrical or unsymmetrical gradual changed in composition from one A block to the other. In the case of the tapered block copolymer of the A-t-B-t-A configuration, the tapered copolymer affords a composition change from A rich to B rich or less A rich then relatively gradually back to A rich. In the case of the tapered block copolymer of the A-B-t-A configuration, the tapered copolymer affords a relatively abrupt composition change from A rich to B rich or less A rich then gradually back to A rich. Conjugated dienes useful in the tapered block copolymers preferably have from about 4 to about 5 carbon atoms, for example, butadiene, isoprene, cis-1,3-pentadiene, trans-1,3-pentadiene, copolymers of two or more dienes such as butadiene and isoprene and block copolymers of two or more such dienes such as butadiene and isoprene.

The tapered block copolymer has a molecular weight of about 10,000 to about 1,000,000 preferably, (for reasons of better performance and handling) from about 50,000 to about 500,000 as determined by gel permeation chromatography.

Particular tapered block copolymers which are useful herein are those taught in U.S. Pat. Nos. 4,427,837, 4,431,777, 4,172,190 and 4,196,154, the teachings of which are specifically incorporated herein by reference.

Tapered A-t-B-t-A block copolymers may be prepared by utilizing difunctional organolithium initiators such as those seen in U.S. Pat. No. 4,196,154. Monovinyl aromatic monomer and conjugated diene monomer are added together for the polymerization. The kinetics of polymerization dictate that the conjugated diene is polymerized first. As the concentration of conjugated diene in the mixture is diminishing gradually during the polymerization reaction, the frequency of monovinyl aromatic monomer entering the chain increases gradually to form the tapered sequence. Eventually only monovinyl aromatic monomer will be left in the mixture to complete the A blocks in the A-t-B-t-A configuration. A typical solvent used for such polymerizations is cyclohexane.

Tapered A-t-B-t-A block copolymers of varying degrees of isolated monovinyl aromatic compound content may be prepared. Such block copolymers made in cyclohexane normally have an isolated monovinyl aromatic monomer content of less than 20% of the total monovinyl aromatic monomer in the tapered block copolymer. Enhanced tapered block copolymers with isolated monovinyl aromatic monomer content of greater than 20% may be prepared by replacing cyclohexane with a more polar hydrocarbon solvent such as toluene, or by the addition of a small amount of a polar aprotic compound to the solvent. Suitable polar aprotic compounds include tertiary amines such as triethyl amine, N,N,N',N'-tetramethylethylenediamine (TMEDA) or N,N,N',N",N"-pentamethyldiethylenetriamine (PMDETA), or ethers such as diethylether or tetrahydrofuran (THF).

Tapered A-B-A block copolymers of the A-B-t-A configuration may be prepared by utilizing a monofunctional organolithium initiators, such as secbutyllithium, to polymerize a quantity of a monovinyl aromatic compound in a suitable polymerization solvent such as cyclohexane first. At the end of the polymerization of the monovinyl aromatic compound and while the propagating chains are still living or reactive, a quantity of a mixture of the monovinyl aromatic compound and a conjugated diene are added to allow the mixture to polymerize onto the existing chain formed during the first polymerization. The first polymerization results in an A block or homopolymer of the monovinyl aromatic compound, and the second polymerization of the mixture completes the tapered A-B-t-A block copolymer. In the second polymerization, the conjugated diene polymerizes preferentially and the monovinyl aromatic compound less preferentially to form a B block having a composition which gradually extends from B rich or less A rich to more A rich or A rich and eventually only monovinyl aromatic monomer will be left to form the remaining A block.

A-B-t-A block copolymers of varying degrees of isolated monovinyl aromatic compound content may be prepared. Synthesis of the A-B-t-A block copolymer in cyclohexane normally results in a block copolymer with an isolated monovinyl aromatic compound content of less than 20% of the total monovinyl aromatic compound based on the weight of the total monovinyl aromatic compound in the block copolymer. The isolated monovinyl aromatic content may be enhanced above 20% by the use of the polar solvents and/or polar aprotic compounds described above. Alternatively, the tapered A-B-t-A block copolymer maybe enhanced by shifting the relative molecular weights of the A blocks from the first A block to the second A block, which is the A block in tapered relationship with the B block. Such shifting may be effected by using less monovinyl aromatic compound for the first polymerization described above and more for the second polymerization involving the polymerization of the mixture of the monovinyl aromatic compound and the conjugated diene.

The bituminous material employed in the practice of the present invention is not critical. Any bitumen or asphalt can be employed. U.S. Pat. No. 3,317,447 contains a good description of useful bituminous materials and the teachings of this patent are specifically incorporated herein by reference. In general, the asphalts which can be employed include conventional petroleum asphalts, natural asphalts, gilsonite, air blown asphalts, coal tar and other similar materials. The asphalts are characterized, for example, by having penetration grades of up to 300 as measured by ASTM Method D5. Asphalts are also characterized by a viscosity grading method, as defined and measured by ASTM 2171, i.e., AC2.5, AC5, AC10, AC20, AC30 and AC40. AC indicates asphalt cement and the number indicates viscosity at 140° F. in poise divided by 1000. Preferred asphalts are those with an AC grade of 40 or less, or penetration numbers of 300 or less. The choice of the degree of penetration will depend on the application for which the blend is to be used. The blend may contain other optional components usually employed in such materials, such as fillers, aggregates, sulfur and the like.

The present blends will preferably comprise about 1 to about 18 weight percent and more preferably about 2 to about 10 percent of the tapered block copolymer based upon the weight of the tapered block copolymer and the binder.

The mixing of the tapered block copolymer and the bitumen takes place under conditions which assure excellent homogeneity. The temperature is usually between 130° and 200° C. so that the components are sufficiently fluid. Furthermore, the mixture must be maintained under agitation for a time sufficient to assure uniform dispersion of the tapered block copolymer into the bitumen. Normally less than 2 hours is sufficient dependent on the physical form of the tapered block copolymer, temperature and mixing intensity. Equipment normally used for heating and mixing materials into asphalt can be employed. The tapered block copolymer may be blended in the composition in emulsion form. In addition to traditional forms such as pellets, crumb, or powder, the block copolymer can be dissolved in a solvent, such as a hydrocarbon, and then blended. In either case, the solvent and water are normally evaporated off during the blending procedure.

Emulsions of block copolymer solutions in water can be prepared by either direct or inverse emulsification techniques. Emulsions may be conveniently prepared from solutions of cyclohexane, toluene, or other hydrocarbon solvents containing 10 to 15 percent by weight polymer and using 2 to 5 percent surfactant by weight based on polymer. Suitable anionic surfactants are, for example, Aerosol OT®, Alipal® CO436, Triton® R770 and mixtures (at about 3 to 1) of Siponate® DS10 (or Calsoft® L40) and Triton® X100 or similar nonionic surfactant. Quaternary ammonium based surfactants such as dodecyltrimethyl ammonium chloride and similar species (including ethoxolated versions) may be used to prepare cationically stabilized emulsions. Best results are often achieved by premixing the surfactant into the polymer solution and then combining with water. For the inversion process, about one volume of water is added at a moderate rate to the polymer-surfactant mixture with continuous moderate agitation, first forming a water-in-oil emulsion which usually inverts to an oil-in-water emulsion when about one half volume of water has been added. If a solvent free dispersion is desired, the solvent may be removed via azeotropic distillation and the resulting latex concentrated to 50 percent or higher solids e.g. by simply distilling out more water.

In the direct emulsification procedure, the polymer solution-surfactant mixture is added to about an equal volume of water with high shear mixing (e.g. Gifford-Wood Homogenizer Mixer) forming the oil-in-water emulsion directly. High shear mixing is continued for a few moments after completion of addition or, alternately, emulsion may be passed through a colloid mill. Solvent-free concentrated latex can be prepared as described above.

Other emulsion forms of the block copolymer are taught in U.S. Pat. Nos. 3,238,172; 3,592,788; 3,565,842; 3,577,250; 3,936,406; 3,957,524; 3,988,275; 4,002,412 and 4,443,084; the teachings of which are specifically incorporated herein by reference.

The blend may contain other optional components usually employed in such materials, such as fillers, aggregates, sulfur and the like.

These bitumen blends may be used for road-coatings, i.e., paving, because of their reduced tendency to rut at elevated temperatures. They are also useful at lower temperatures because of a reduced tendency to crack and ravel. These bitumen blends also exhibit a strong elasticity, a high plasticity interval, which means that they may also be used to advantage in the manufacture of industrial coatings (coatings, superficial coats, asphalt covers) or for industrial applications such as coverings, felts, shingles, paints, and products for the protection of car bottoms. They may also be used as crack sealants, chip sealants, and the like.

The present blends of bituminous material and tapered block copolymers have improved toughness and elasticity compared to prior art blends. Present blends utilizing enhanced tapered block copolymers have even more improved toughness and elasticity compared to prior art blends.

The following examples are provided for illustrative purposes and are not intended to limit the invention to the specific embodiments set forth therein.

EXAMPLE 1

Bituminous blends of the present invention utilizing tapered A-B-A block copolymers of the A-t-B-t-A configuration were prepared as outlined in Procedures A-C below and tested for physical properties relevant to asphalt applications. Results of the tests are set forth in Tables 1 and 2.

PROCEDURE A

A tapered A-B-A block copolymer of the A-t-b-t-A configuration useful in the blends of the present invention was prepared. To a nitrogen purged 2 liter air-tight stirred reactor, 1400 mL purified toluene was added. To the same vessel, 230 mL of purified 1,3-butadiene and 80.6 mL of purified styrene were added. A 60 mL quantity of this reaction mixture was withdrawn from the reaction vessel and titrated with a toluene solution of 1,3-phenylene-bis(3-methyl-1-phenylpentylidene)-bis-(lithium), a difunctional organolithium initiator prepared as in U.S. Pat. No. 4,196,154 (the teachings of which are incorporated herein by reference) until a faint reddish color appeared. Based on the equivalency of the difunctional initiator used in the titration, an equivalent amount of s-butyllithium solution was added to the reaction mixture to blank out the impurities. The reaction mixture in the reactor was then heated to 50° C. and 1.78 millimoles of the above difunctional organo-lithium initiator solution was added to initiate the polymerization of butadiene. The temperature of the reaction mixture rose to about 80° C. in about 45 minutes after initiation. Shortly thereafter the faint yellowing solution turned to a more intense red color indicating that styrene polymerization had started. A second rise in temperature to about 84° C. was observed. Afterwards the temperature started to subside. The polymerization wa allowed to proceed for 30 minutes after the color change and then was terminated by the addition of isopropyl alcohol. The recovered block copolymer was a tapered SBS triblock copolymer. A small portion of the polymer was recovered from the toluene solution for the molecular weight determination by gel permeation chromatography (GPC) (employing Millipore-Waters 150C instrument with 6 TSK columns). The molecular weight at the peak of the distribution as determined by GPC was 133,000. The polymer based on the feed used had 67 percent by weight butadiene and 33 percent by weight styrene.

PROCEDURE B

A tapered A-B-A block copolymer of the A-t-b-t-A configuration useful in the blends of the present invention was prepared. In making a tapered block polymer the same conditions were used as that in Procedure A except that after the polymerization the reactive anions were carefully terminated by adding isopropyl alcohol dropwise until the red polystyryl anion color just disappeared. A 100 mL portion of syrup was withdrawn from the reactor for analysis. Afterwards another 230 mL of butadiene and 80.6 mL of styrene were added and the polymerization was repeated. The final syrup was saved and combined with another syrup made in exactly the same manner. The final polymer was a tapered SBS with a molecular weight of 131,000. The styrene content was 33% by weight and butadiene 67%.

PROCEDURE C

A tapered A-B-A block copolymer of the A-t-b-t-A configuration useful in the blends of the present invention was prepared. The procedure used was similar to that of Procedure A with the exception that isopropoxide was used as a coinitiator together with the initiator used in preparing Polymer A. This coinitiator system is described in copending patent application Ser. No. 913,927 filed Oct. 1, 1986. The molecular weight distribution of the block copolymer made is narrower than the block copolymer made without the isopropoxide. The polymer had a molecular weight of 150,000 at the peak of the GPC chromatogram and the Mw/Mn ratio for the block copolymer was 1.08. The isopropoxide was generated in situ by the reaction of isopropanol (isopropyl alcohol) with the blanking s-butyllithium. The 1/10 isopropanol/toluene was made in a 10 mL volumetric flask with about 1 mL isopropanol and toluene of sufficient quantity to make up the 10 mL volume. The concentration was therefore about 1.31N. The amount of isopropoxide generated was about 1.35 milliequivalents.

The procedures A-C, were employed to make various tapered block copolymers having different molecular weights and styrene content. The tapered block copolymers were added in various forms to an asphalt (Marathon AC-10: 85-100 penetration number) and various characteristics of the blend determined. The tapered block polymer was added as a solution in toluene, as solid particles and as an anionic emulsion in water. The solution form consisted of the tapered block copolymer in toluene as formed in the polymerization reaction. The emulsion was formed by adding an anionic surfactant and water to the toluene solution of the tapered block copolymer and emulsifying the blend by running it through a collodial mill or high shear mixer. The toluene in the emulsion was then steam stripped under a vacuum and then the emulsion was concentrated to greater than 50 percent by weight solids. Solid tapered block copolymer was formed by drying the toluene solution in a vacuum oven and cutting the solid tapered block copolymer into pellets.

The tapered block copolymer was mixed with the asphalt by throughly blending the solution, solid, or emulsion with the asphalt while at a temperature of about 130°-140° C. and then continuing the mixing at an elevated temperature of 185°-190° C. for 1-2 hours to evaporate all toluene and water as the case may be. The modified asphalt was then cast into test strips and subjected to one or more of the following tests, and the results set forth in Tables 1 and 2.

A. Percent elongation was measured on an Instron test machine at a temperature of -10° C. and 4° C. at a pull rate of 0.1 inch/minute and 2 inches/minute respectively. The results of these tests are identified in the Table as Elongation (%). The test is similar to the ductility test except the results are determined with the Instron machine instead of a ductilometer.

B. Tensile strength (psi) was determined by using data obtained using an Instron Model No. 1125 instrument.

C. Ductility (cm) was determined according to ASSHTO method T51-81. This test measures the distance a sample will elongate before breaking when two ends of a briquet sample are pulled apart at a specified speed and temperature. The data set forth in the following table reports the high and low values determined for a number of test specimens. It's not unusual for such variance in results employing this test procedure.

D. Elastic recovery (%) was determined by measuring the amount of recovery after 10 centimeters extension in the ductility test.

E. Toughness and tenacity ((lbs./sq.-in.)-in.)) was measured using the Benson test (test proposed by J. R. Benson, Consulting Bituminous Engineer of Denver, Colo. in 1955, reported in Utah State Specifications for Rubberized Asphalt). This test indicates the toughness of the modified asphalt as measured by the inch-pounds of work required to separate a tension head and cup holding the sample in the test apparatus.

F. Torsional recovery (%) is the test known in the art as California test 332 (CT 332). This test measures the amount of elasticity that is imparted to an asphalt residuum that was treated with an elastomeric latex or emulsion.

                                      TABLE 1                                      __________________________________________________________________________                                             Tensile Strength                                                                        Tensile Properties                                         Wt. %      10° C. @ 0.1                                                                     4° C. @ 2"/min         Test                                                                              Polymerization                                                                          Mol Wt. at                                                                           Weight Av. Polymer in                                                                           Form of                                                                             Ty Elongation                                                                           Ty Elongation                 No.                                                                               Procedure                                                                               GPC Peak                                                                             Mol Wt.                                                                              % STY                                                                               Asphalt                                                                              Polymer                                                                             (psi)                                                                             %     (psi)                                                                             %                          __________________________________________________________________________     1  C        131,000                                                                              151,000                                                                              33   5     solution                                                                            300                                                                               NB     90                                                                               NB                         2  A        133,000                                                                              146,000                                                                              33   5     solution                                                                            350                                                                               NB     90                                                                               NB                         3  C        131,000                                                                              151,000                                                                              33   5     emulsion                                                                            600                                                                               NB    200                                                                               NB                         4  A        137,000                                                                              130,000                                                                              33   5     pellets                                                                             200                                                                               NB                                  5  A        123,000                                                                              133,000                                                                              33   5     pellets                                                                              45                                                                               NB                                  6  None     --    --    --   0      --  100                                                                               0.3   200                                                                               25                            (Marathon AC-10                                                                Asphalt Alone)                                                              __________________________________________________________________________      NB = did not break at limit of test equipment  1400 cm.                  

                                      TABLE 2                                      __________________________________________________________________________                Mol Wt                                                                              Weight  Wt %             Elastic                                                                              Toughness/                      Test                                                                              Polymerization                                                                         at GPC                                                                              Av. Mol                                                                             %  Polymer in                                                                           Form of                                                                             Ductility                                                                            Recovery                                                                             Tenacity                        No.                                                                               Procedure                                                                              Peak Wt.  STY                                                                               Asphalt                                                                              Polymer                                                                             (cm)  %     (lbs/sq.in)-in                  __________________________________________________________________________     1  A       135,000                                                                             151,000                                                                             33 5     solution                                                                            24-74 --                                    2  A       220,000                                                                             237,000                                                                             33 5     solution                                                                            53.5-56                                                                              --                                    3  A       161,000                                                                             191,000                                                                             33 5     solution                                                                            50.5-55                                                                              --                                    4  A       133,000                                                                             146,000                                                                             33 5     solution                                                                            83.5-132.5                                                                           --                                    5  A       123,000                                                                             133,000                                                                             33 5     solution                                                                            63.5-73                                                                              --                                    6  A       137,000                                                                             132,000                                                                             25 5     solution                                                                            64-75 --                                    7  A       137,000                                                                             130,000                                                                             33 5     solution                                                                            52-54 --                                    8  C       131,000                                                                             151,000                                                                             33 5     solution                                                                            75-87 --                                    9  C       131,000                                                                             151,000                                                                             33 5     emulsion                                                                            72-81 --                                    10 C       131,000                                                                             151,000                                                                             33 5     powder                                                                              53-55 --                                    11 A       160,000                                                                             182,000                                                                             33 3     emulsion                                                                            35.4  63.3  201/150                         12 B       150,000                                                                             153,000                                                                             33 3     emulsion                                                                            29.6  65.0  168/113                         13 A       146,000                                                                             160,000                                                                             33 3     emulsion                                                                            28.5  62.5  132/88                          14 A       204,000                                                                             266,000                                                                             33 3     emulsion                                                                            48.8  67.5  241/176                         15 A       154,000                                                                             190,000                                                                             38 3     emulsion                                                                            27.6  67.5  239/178                         16 A       171,000                                                                             217,000                                                                             28 3     emulsion                                                                            34.6  65.0  171/119                         __________________________________________________________________________

Test 6 in Table 1 was a control which did not employ tapered block copolymers of the invention. Test 6 employed 100 percent Marathon AC-10 asphalt.

The results set forth in Tables 1 and 2 demonstrate that the tapered SBS component imparts high elongation characterization to the composition of the invention in comparision to the untreated asphalt. High elongation implies that the resulting asphalt-copolymer blend is more resistant to fracture under strain than the untreated asphalt.

COMPARATIVE EXAMPLE

Bituminous blends of the present invention utilizing tapered A-B-A block copolymers of the A-t-B-t-A configuration were prepared as outlined in Procedures D and E below with Procedure E yielding an enhanced tapered block copolymer and Procedure D yielding a tapered block copolymer which was not enhanced Blends utilizing the two different polymers were compared on the basis of physical properties relevant to asphalt applications. Results of the tests are set forth in Table 3.

PROCEDURE D

A tapered A-B-A block copolymer of the A-t-B-t-A configuration useful in the blends of the present invention was prepared. To a nitrogen purged 2 liter air-tight stirred reactor, 1350 ML purified cyclohexane was added. To the same vessel, 222 mL of purified 1,3-butadiene and 77.8 mL of purified styrene were added. The reaction mixture in the reactor was then heated to 50° C. and 1.80 millimoles of a difunctional organolithium initiator solution was added to initiate the butadiene styrene copolymerization. The difunctional initiator was a toluene solution of 1,3-phenylene-bis(3-methyl-1-phenylpentylidene)-bis-(lithium), prepared as described in U.S. Pat. No. 4,196,154. At the initial stage the dark red initiator color quickly changed to a pale yellow indicating that the monomer being polymerized was mostly butadiene. The temperature of the reaction mixture rose to about 73° C. in about 1 hour after initiation. Shortly thereafter the faint yellowing solution turned to a more intense red color indicating that styrene polymerization had started. A second rise of temperature was observed. The polymerization was allowed to proceed for 30 minutes after the color change and then was terminated by the addition of isopropyl alcohol. The block copolymer was a tapered-SBS triblock polymer. The weight average molecular weight as determined by gpc was 178,000. The result of proton nmr analysis showed the polymer had 64.2% butadiene and 35.8% styrene 16.7% of which was isolated styrene. The blend utilizing this tapered block copolymer was labeled as Sample No. 1.

PROCEDURE E

An enhanced tapered A-B-A block copolymer of the A-t-B-t-A configuration useful in the blends of the present invention was prepared. The polymerization in Procedure D was repeated with the exception that 1.24 millimoles of initiator was used and 0.062 millimoles of PMDETA was added just before the addition of the initiator. The tapered-SBS obtained has a weight average molecular weight of 293,000 by gpc and a composition of 69.3% butadiene and 29.3% styrene. The amount of isolated styrene was enhanced to 29.3% from about 17% as a result of the added tertiary-triamine. The blend utilizing this tapered block copolymer was labeled as Sample No. 2.

The recovered polymer samples from Procedures D and E were dissolved in toluene. The toluene solutions of the two tapered-SBS were then emulsified in water utilizing a surfactant and a colloid mill. Toluene in the emulsion was then driven away by treatment with steam under a vacuum and the emulsions were then concentrated by evaporation to greater than 50% solid. Each of the emulsions was added to a hot Marathon AC-10 asphalt with a Jiffy mixer. After heating to 180°-190° C. to assure complete evaporation of water, the modified asphalts with 5% of tapered-SBS were tested for ductility at 39° F. by test method ASSHTO T 51-81 described above and for toughness and tenacity by the Benson test described above. Test results for a blend not of the present invention utilizing a conventional untapered A-B-A block copolymer (Kraton 1101 marketed by the Shell Chemical Company) is also shown. The blend utilizing the untapered block copolymer was prepared in substantially the same manner as those of Samples 1 and 2.

                  TABLE 3                                                          ______________________________________                                                  Isolated                Toughness/                                             styrene in    Ductility tenacity                                      Sample No.                                                                              copolymer (%) 39° F. (cm)                                                                       (lbs/sq. in)-in                               ______________________________________                                         Sample 1 16.7          29.3      232/152                                       Sample 2 29.3          43.4      236/187                                       Kraton 1101                                                                             0             17.8      113/43                                        ______________________________________                                    

Sample 1, a blend of the present invention, demonstrated superior ductility and toughness/tenacity compared to the blend utilizing the untapered A-B-A block copolymer (Sample 3). Sample 2, also a blend of the present invention, utilizes a tapered A-B-A block copolymer of a higher isolated styrene content than that of Sample 1, showed superior ductility and toughness/tenacity compared to the blend of Sample 1.

EXAMPLE 2

Blends of the present invention utilizing tapered block copolymers prepared according to Procedure E were prepared as outlined in the Comparative Example except that the type and/or amount of initiator and aprotic additive used was varied. The blends utilized 3 weight percent tapered block copolymer based upon the weight of the binder and the copolymer. Ductility was measured according to AASHTO T51-81 described above and melt flow rate according to condition 200/5.0 ASTM method D 1238-82. A blend utilizing Kraton 1101 (described above) and straight asphalt (Baytown asphalt by Exxon Chemical Company) were also tested. Physical properties of the blends are shown in Table 4.

                                      TABLE 4                                      __________________________________________________________________________              Additive                                                                             Addit Conc     Melt Flow   Ductility                            Sample   Type  mol/Li (c)                                                                           Mw  % sty (a)                                                                           Rate  i-sty % (b)                                                                          (cm)                                 __________________________________________________________________________     Sample 1 THF   6.4   193000                                                                             33.5 5.6   74.6  >150                                 Sample 2 PMDETA                                                                               0.15  148000                                                                             33.6 5.1   59.5  71                                   Sample 3 PMDETA                                                                               .075   45000                                                                             34.6 1.4   46.3  75                                   Sample 4 THF   4.27  249000                                                                             33.9 1.1   69.1  81                                   Sample 5 PMDETA                                                                               --    160000                                                                             40.1 3.7   65.5  34                                   Sample 6 PMDETA                                                                               --    133000                                                                             35.7 0.4   43.4  66                                   Sample 7 THF   12.8  263000                                                                             40.2 1.6   73.0  65                                   Kraton ® 1101        30.0 0.0   0.0   25                                   Baytown asphalt                           16                                   __________________________________________________________________________      (a) % styTotal weight styrene in tapered copolymer                             (b) isty %weight % isolated styrene in tapered copolymer                       (c) concentration expressed as moles of additive per mole of Li in the         initiator used.                                                          

EXAMPLE 3

A typical tapered A-B-A block copolymer of the A-t-B-t-A configuration having a GPC peak molecular weight of 225,000 was prepared according to procedure A. Also prepared was a 62 percent solids aqueous emulsion of the SBS copolymer as taught herein. These solutions and emulsions were mixed with Ergon Roofing Grade asphalt.

In preparing the emulsion-based compositions, the emulsion was added dropwise to impeller stirred asphalt heated to 160° to 170° C. to form test samples having tapered block copolymer proportions ranging from 6 to 16 percent by weight. The temperature during mixing dropped to 130° to 140° C. due to water evaporation. The temperature was increased eventually to 180° C. and the samples finished in the vacuum oven at 165° C. for 11/2 hours, followed by remixing and 2 more hours in the oven. The resulting samples were tested for toughness and tenacity with the results appearing in Table 5.

                                      TABLE 5                                      __________________________________________________________________________     Test                  % SBS                                                    No.  Description of Sample                                                                           based on SBS + asphalt                                                                     To.sup.1                                                                           Te.sup.2                                 __________________________________________________________________________     Control                                                                             Ergon asphalt alone                                                                              0          18.5                                                                               13                                       A    62% SBS solids/aqueous-emulsion                                                                  6          99  83                                       B    62% SBS solids/aqueous-emulsion                                                                 10          162 141                                      C    62% SBS solids/aqueous-emulsion                                                                 16          198 *                                        __________________________________________________________________________      .sup.1 To = Toughness: The entire area of a stressstrain curve obtained o      a ductilometer at 4° C. and 5 cm/min. pull rate. ASTM D113.             .sup.2 Te = Tenacity: The area under the stressstrain curve attributable       to elastomer modifier SBS.                                                     *No yield point on stressstrain curve.                                   

PREPARATION OF A-B-t-A BLOCK COPOLYMERS PROCEDURE F

An tapered A-B-A block copolymer of the A-B-t-A configuration useful in the blends of the present invention was prepared. To a nitrogen purged 2 liter air-tight stirred reactor, 1350 mL purified cyclohexane was added. To the same vessel, 33.5 mL of purified styrene were added. The reaction mixture in the reactor was then heated to 55° C. and 3.12 millimoles of secbutyllithium in cyclohexane solution was added to initiate styrene polymerization. After 30 minutes when styrene polymerization became essentially completed, 146 grams of butadiene premixed with 33.5 mL of purified styrene was added. Upon the addition, the strong dark orange color of the reaction mixture faded immediately until after most of the butadiene monomer was polymerized. The color of the faint yellowing solution turned once again to a more intense red color indicating that the second styrene polymerization has started. The polymerization was allowed to proceed for 30 minutes after the color change and then was terminated by the addition of isopropyl alcohol. The block copolymer was a half tapered-SBS triblock polymer. Proton nmr analysis showed the polymer had 69.1% butadiene and 30.9% styrene 14.2% of which was isolated styrene.

PROCEDURE G

An enhanced tapered A-B-A block copolymer of the A-B-t-A configuration useful in the blends of the present invention was prepared. Procedure F was repeated except that 25 mL of styrene monomer was used in the preparation of the first polystyrene block and the 42 mL of styrene was mixed with butadiene for the second monomer addition. The half tapered-SBS recovered had 69.9% butadiene and 30.1% styrene, 22.8% of which was isolated styrene.

PROCEDURE H

An enhanced tapered A-B-A block copolymer of the A-B-t-A configuration useful in the blends of the present invention was prepared. Procedure F was repeated except that 2.0 instead of 3.12 millimoles of sec-butyllithium was used as the initiator and 0.48 millimoles of PMDETA was used as a polar additive. The half tapered-SBS recovered has 69.7% butadiene and 30.3%, styrene 49.5% of which was isolated styrene.

PROCEDURE I

An enhanced tapered A-B-A block copolymer of the A-B-t-A configuration useful in the blends of the present invention was prepared. Procedure G was repeated except that 2.37 instead of 3.12 millimoles of sec-butyllithium was used as the initiator and 0.57 millimoles of PMDETA was used as a polar additive. The half tapered-SBS recovered had 69.5% butadiene and 30.5% styrene, 64.0% of which was isolated styrene. 

What is claimed is:
 1. A tapered block copolymer of the A-t-B-t-A or A-B-t-A configuration, the A blocks being formed from a monovinyl aromatic compound and the B block being formed from butadiene wherein between 29 and 70 weight percent of the total monovinyl aromatic compound in the tapered block copolymer is isolated.
 2. A taper ed block copolymer according to claim 1 wherein the monovinyl aromatic compound is styrene. 